Network Working Group N. Freed
Request for Comments: 2978 Innosoft
BCP: 19 J. Postel
Obsoletes: 2278 ISI
Category: Best Current Practice October 2000
IANA Charset Registration Procedures
Status of this Memo
This document specifies an Internet Best Current Practices for the
Internet Community, and requests discussion and suggestions for
improvements. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2000). All Rights Reserved.
Abstract
Multipurpose Internet Mail Extensions (MIME) (RFC-2045, RFC-2046,
RFC-2047, RFC-2184) and various other Internet protocols are capable
of using many different charsets. This in turn means that the
ability to label different charsets is essential.
Note: The charset registration procedure exists solely to associate a
specific name or names with a given charset and to give an indication
of whether or not a given charset can be used in MIME text objects.
In particular, the general applicability and appropriateness of a
given registered charset to a particular application is a protocol
issue, not a registration issue, and is not dealt with by this
registration procedure.
1. Definitions and Notation
The following sections define terms used in this document.
1.1. Requirements Notation
This document occasionally uses terms that appear in capital letters.
When the terms "MUST", "SHOULD", "MUST NOT", "SHOULD NOT", and "MAY"
appear capitalized, they are being used to indicate particular
requirements of this specification. A discussion of the meanings of
these terms appears in [RFC-2119].
Freed & Postel Best Current Practice [Page 1]RFC 2978 IANA Charset Registration Procedures October 20001.2. Character
A member of a set of elements used for the organization, control, or
representation of data.
1.3. Charset
The term "charset" (referred to as a "character set" in previous
versions of this document) is used here to refer to a method of
converting a sequence of octets into a sequence of characters. This
conversion may also optionally produce additional control information
such as directionality indicators.
Note that unconditional and unambiguous conversion in the other
direction is not required, in that not all characters may be
representable by a given charset and a charset may provide more than
one sequence of octets to represent a particular sequence of
characters.
This definition is intended to allow charsets to be defined in a
variety of different ways, from simple single-table mappings such as
US-ASCII to complex table switching methods such as those that use
ISO 2022's techniques. However, the definition associated with a
charset name must fully specify the mapping to be performed. In
particular, use of external profiling information to determine the
exact mapping is not permitted.
HISTORICAL NOTE: The term "character set" was originally used in MIME
to describe such straightforward schemes as US-ASCII and ISO-8859-1
which consist of a small set of characters and a simple one-to-one
mapping from single octets to single characters. Multi-octet
character encoding schemes and switching techniques make the
situation much more complex. As such, the definition of this term
was revised to emphasize both the conversion aspect of the process,
and the term itself has been changed to "charset" to emphasize that
it is not, after all, just a set of characters. A discussion of
these issues as well as specification of standard terminology for use
in the IETF appears in RFC 2130.
1.4. Coded Character Set
A Coded Character Set (CCS) is a one-to-one mapping from a set of
abstract characters to a set of integers. Examples of coded
character sets are ISO 10646 [ISO-10646], US-ASCII [US-ASCII], and
the ISO-8859 series [ISO-8859].
Freed & Postel Best Current Practice [Page 2]RFC 2978 IANA Charset Registration Procedures October 20001.5. Character Encoding Scheme
A Character Encoding Scheme (CES) is a mapping from a Coded Character
Set or several coded character sets to a set of octet sequences. A
given CES is sometimes associated with a single CCS; for example,